Built in Doncaster in 1938, Mallard was one of thirty-five A4 Pacific class locomotives designed by Sir Nigel Gresley for London and North Eastern Railway (LNER).

The A4 class were introduced in 1935. The first batch consisted of four locomotives which were painted silver-grey, the first of which was "Silver Link". By 1938 the standard colour for streamlined LNER locomotives was garter blue, with locomotives being repainted as they came through workshops for maintenance.

On 3 July 1938, during a series of brake tests, Mallard achieved a record breaking speed of 126 mph at Stoke Bank, between Grantham and Peterborough. According to the driver Joe Duddington, Mallard ran consistently at 125 mph and peaked at 126 mph for approximately a quarter of a mile, although he believed it was capable of reaching 130 mph. It is believed that Gresley planned to make another run in September 1939 in order to reach 130 mph, but the Second World War prevented this attempt.

In 1938 LNER and Gresley were only prepared to publicise a record of 125mph (beating the previous steam record of 114mph – non-steam locomotives had already achieved greater speeds). The world speed record plaque which records the speed as 126mph was put on the locomotive in 1948, after Gresley’s death in 1941.

Mallard retired from service in 1963 and was subsequently preserved in 1964 by the British Transport Commission. Mallard entered the National Railway Museum’s collection in 1975. It was restored to working order at York, between 1982 and 1988 and completed a limited number of runs until 1989.

Mallard is now one of only six remaining A4 Pacifics. In 2013, to celebrate the 75th anniversary of Mallard’s speed record, all six locomotives were reunited at the National Railway Museum. Today Mallard is one of the highlights of the museum.